Conventionally, television audio signals are transmitted by frequency modulating an audio carrier having a frequency within a transmission band of a broadcasting channel, wherein the modulated audio signal is transmitted along with a modulated video signal and is received and demodulated at a receiver. Such a system is used, for example, by the National Television Systems Committee (NTSC) system.
Also known is a system of high fidelity audio signal transmission which utilizes pulse code modulation (PCM) to encode an audio signal and time multiplexes the PCM audio signal within suitable intervals of a video signal, as disclosed for a high definition television system such as the so-called MUSE system disclosed in U.S. Pat. No. 4,745,459.
The FM system exhibits a high immunity to noise in transmission. However, the FM system is more prone to distortion in transmission than PCM, and thereby offers lower fidelity than the PCM system. While offering higher fidelity than conventional FM transmission, the PCM system deteriorates rapidly when the signal-to-noise ratio is low over a particular transmission channel.